How To

Formwork Pressure Calculations

Q.: American Concrete Institute formulas for lateral pressure of concrete (ACI 347-78, Section 2.2) show that pressure is inversely related to the temperature of the concrete in the forms. However, the temperature of the concrete is rarely (if ever) monitored in the forms. In cold weather, concrete may be heated to 60 to 70 degrees F when mixed. It is then placed in steel or wood forms which are at air temperature. The placement from bottom of form to top can easily occur over a period of 1 to 5 hours. There is an initial heat loss to the cold forms and a continuing heat loss through steel or plywood. Can one assume an appropriate concrete temperature for the formulas or should ambient temperature be used for pressure computation?

A.: There is nothing in the ACI 347 Standard itself that answers this question. Some heat is generated in the concrete as the cement hydrates. However, the effect of this heat varies greatly, depending on job conditions. Form material, thickness of concrete cross section, wind, condition of forms and type of external protection all have an influence.. The effect of cold forms and heat loss through the forms would slow down the rate of hardening, thus keeping the internal pressure on the forms high for a longer period of time.

This is the same effect a retarding admixture would have, though not necessarily of the same magnitude. For this reason it might be well to follow the recommendation:

"Where form temperature is less than that of the concrete, an effective value of temperature less than that of the concrete in the forms should be used in the formulas."

This is an adaptation of a recommendation given in Section 2.2.2 (f) of the Standard. (That section concerns retarding admixtures but not temperatures.) In effect this advice counsels making an informed guess about how much lower a temperature value should be used in the formulas. One could assume an appropriately low concrete temperature. It would be better still to measure it. Or, to be completely conservative, the ambient temperature could be used, provided it is 40 degrees F or more. If the ambient temperature is below 40 degrees F and the concrete is not protected against cold weather, you could calculate the hydrostatic pressure in pounds per square foot by multiplying the height of the concrete in the forms by 150.

There may be further problems if use of fly ash or retarding admixtures is intended. The use of these materials in cold weather should be seriously questioned because they prolong the rate of setting. If they must be used, extend the length of time before the forms are stripped.

The ACI pressure formulas were developed on the basis of actual experience, using information for temperatures which did not go below 40 degrees F. Winter concreting recommendations such as those published by ACI Committee 306 require concrete to be 40 degrees F or higher when placed and to be protected from freezing for some minimum period of time. Without this required protection, concrete conceivably could freeze at the time of placement (relieving lateral pressure of course), then thaw later and overload the forms or lead to collapse of the structure. This actually happened during the early days of concrete building construction, as Jacob Feld explained in the article "Temperature Effects on Completed Work and During Construction", Concrete Construction, March 1964, page 61.